1. Field of the Invention
The present invention relates to an electrical filter, in particular to an electrical filter which can be used for filtering external disturbances in differential data lines.
2. Description of Related Art
In systems using differential data lines for transmitting signals, a considerable problem is filtering out external disturbances in the broadest band possible. Examples of this are disturbances which are, for example, caused by the fact that strong transmitters couple in the data lines so that these disturbances generally occur as common mode signals. Common mode signal in this context means a signal which is present on at least two signal or data lines in a common mode. Signals which occur in a common mode here are signals which, in the ideal case, exist on the different data lines without any phase shift or temporal shift. However, in real cases, such signals which have a phase shift of a magnitude of less than 30°, preferably less than 10° to one another, will still be referred to as common mode signals. In contrast to disturbances, data are transported via differential data lines in the form of differential signals, i.e. in the form of essentially anti-phase signals. An anti-phase or differential signal in the present application is a signal which, in the ideal case, has a phase shift of 180° on the different data lines. However, in real cases, even such signals having a phase shift of 150° to 210°, preferably 170° to 190° to one another will still be referred to as being anti-phase or differential.
Thus, the frequency of the differential signal may even be higher than the frequency of the external disturbance or the respective disturbance signal.
Common mode noise filters, for example, are required at present to block disturbances caused by mobile phones at USB interfaces. Other fields of application of these common mode noise filters are in the field of differential high-speed transmission circuits for transmitting signals operating according to the USB 2.0 standard already mentioned, also according to the IEEE 1394 standard known as Firewire® in wire-based networks, and according to different LVDS (low voltage differential signaling) standards. Such filters are, for example, applied in the field of signal transmission according to the USB 2.0 and IEEE 1394 standards in small mobile apparatuses, such as, for example, digital cameras, digital video recorders, notebooks, PDAs (personal data assistants) and PCs. In the field of other LVDS data lines, such filters are, for example, employed when transmitting signals between notebooks, PCs and LCD (liquid crystal display) screens and other peripherals operating according to an LVDS standard.
According to the conventional art, such signals are realized by discrete, comparatively large and expensive transformers using ferrites to increase inductance. Examples of such filters are EXC24CD and EXC28CE-type filters by Panasonic® or Matsushita Electronic Components Co., Ltd., the technical data and field of application of which are contained in the respective data sheets and a press report which can be looked at on the homepage of Matsushita Electronic Components Co., Ltd. on the day of application. Another example of a USB 2.0 common mode filter is the 1202 filter family by Coilcraft® Inc. company, the technical data of which are also specified in the technical data sheets which can for example be looked at on Coilcraft®'s homepage on the day of application.
Even if today's solutions exhibit a relatively low direct current resistance, however, the disadvantages of these solutions exceed this advantage by far. The disadvantages of this solution in particular are that the difference in attenuation for the common mode and the differential mode is relatively low and that the attenuation of the differential mode is reduced considerably with higher frequencies, i.e. for example in a frequency range above 1 GHz, which becomes evident from measurements of the typical attenuation as a function of the frequency in the common mode and the differential mode. Additionally, the frequency range of the differential signal usable is limited by the self-resonance of the transformer and/or the ferrite material used. As another disadvantage, frequently additional costs are caused due to the fact that measures must be taken for a separate ESD (electrostatic discharge) protection against electrostatic discharges since these filters frequently are located at the interfaces of PCs or other apparatuses from the field of consumer electronics and telecommunications. Even realizing a multi-stage filter concept of the conventional art is of particular disadvantage since in this case several, comparatively large devices have to be connected in series. Frequently, especially in the field of mobile applications, there is no space available for this.
FIG. 5a illustrates a circuit diagram of a transformer filter 800. A first inductance 810 of an electrical inductance value of L1=25 nH is connected between a first terminal 800a and a second terminal 800b, a second inductance 820 of an electrical inductance value of L2=25 nH is connected between a third terminal 800c and a fourth terminal 800d. The first inductance 810 and the second inductance 820 here form a transformer, the winding arrangement of which is illustrated by two black points and selected such that a current flowing through the first inductance 810 causes a magnetic flux of the same sign in the second inductance 820. Due to this winding orientation of the two inductances 810, 820, the two inductances 810, 820 are also referred to as being positively coupled.
The transformer here has a coupling coefficient K of around 1. The coupling coefficient K here is defined as the ratio of a mutual inductance M of one of the two inductances 810, 820 referenced to the respective other of the inductances 810, 820 and the square root of the product of the two values L1 and L2 of the two inductances 810, 820. Thus, the following applies:
  K  =      M                  L        ⁢                                  ⁢                  1          ·          L                ⁢                                  ⁢        2            
Thus, the transformer has a total conductivity or effective conductivity Ltot depending on the coupling coefficient K fulfilling the following relation:Ltot=L1+L2+2M relative to the mutual inductance M depending on the coupling coefficient K.
According to the conventional art, an inductive filter which is also referred to as L filter is used. A coupled coil pair and/or a transformer of high inductance is used for filtering, wherein the coupling of the coils and/or inductances in the transformer is close to 1. In the common mode, the transformer thus represents a low-pass filter since in this mode the inductances are positive-coupling so that the impedance, too, becomes very high due to the high resulting effective inductance, as does the resulting attenuation. For the differential mode, the inductances are negative-coupling so that the effective impedance and the resulting attenuation become very low.
The inductances of the differential mode and the common mode on the one hand differ by the effective connection which results in halving in the common mode due to the resulting effective parallel-connection of the inductances and in doubling in the differential mode due to the effective series-connection of the inductances and, on the other hand, due to coupling, wherein there is an effective coupling coefficient of K<0 in the differential mode and an effective coupling coefficient of K>0 in the common mode. Referring to the values L1 and L2 of the two inductances 810, 820 shown in FIG. 5, the result in the common mode is an effective inductance L_CM=100 nH as a total inductance Ltot and in the differential mode an effective inductance L_DM of 0 nH as a total inductance Ltot.
FIG. 5b shows a plotting of a numerically established attenuation S12 as a function of a frequency F of a signal coupled in for different signal modes and different effective inductances for the circuit diagram of a filter 800 illustrated in FIG. 5a. The attenuation forms shown in FIG. 5b are based on a viewing limited to an ideal case where parasitic effects, as may, for example, be caused by (parasitic) inductances, (parasitic) capacitances and/or (parasitic) resistances, are not taken into consideration. In particular, FIG. 5b shows an attenuation form 900 resulting in the case of negative coupling for the filter 800 shown in FIG. 5a and which in FIG. 5b is also referred to as DM standing for differential mode. The attenuation form 900 shows no dependence on the frequency F, but monotonously stays at a value of about 0 dB. The reason for this is the nearly disappearing effective inductance L_DM of the filter 800 shown in FIG. 5a in the differential mode, with which the impedance and thus attenuation of the filter 800 also disappear. In addition, FIG. 5b shows an attenuation form 910 for the filter 800 shown in FIG. 5a in the common mode for an effective inductance L_CM=100 nH which in FIG. 5b is also referred to as CM standing for common mode. With an increasing frequency F starting from an attenuation value of around 0 dB, the attenuation form 910 exhibits a monotonously decreasing form. Based on an effective inductance in the common mode of L_CM=100 nH, exemplarily the result is an effective attenuation of around 11 dB at a frequency of around 1 GHz.
Additionally, to demonstrate that a very high effective inductance L_CM is required in such an L filter having a coupling coefficient K of approximately 1 in order to achieve a very high attenuation in the common mode (CM attenuation), FIG. 5b shows an attenuation form 920 which is based on an effective inductance in the common mode of L_CM=800 nH, i.e. in the case of a symmetrical division of the inductances L1 and L2 of the two inductances 810 and 820 of 200 nH each. It is possible by this increase in the values L1 and L2 of the inductances 810 and 820 to, for example, achieve an attenuation of around 29 dB at around 1 GHz, however, this requires a considerable increase in the inductances L1 and L2 of the two inductances 810, 820 of the transformer of the filter shown in FIG. 5a. Such an increase generally also causes a considerable increase in the space required which frequently, in particular in mobile applications, is not available.
There is a need, therefore, for an electrical filter having an improved filter characteristic for common mode signals and differential signals.